<?php	
//1.检索出标题对应的标签id
select a.atitle,c.tid from arc a join center c on a.aid=c.tid;
//2.检索出每个标题所对应的标签名称
select a.atitle,t.tname from arc a join cencer c on a.aid=c.aid join tag t on c.tid=t.tid;
//3.检索出每个标签对应的文章数量
select t.tname,count(*) from arc a join center c on a.aid=c.aid join tag
t on c.tid=t.tid group by t.tname;









alter table stu modify name varchar(30) not null default '';
alter table stu modify sid int unsigned;
alter table stu drop primary key;
alter table stu add primary key(sid);
alter table stu modify sid int unsigned auto_increment;
